1. Field of the Invention
The present invention relates to an objective lens (a photographic lens) for use in an optical apparatus, such as a telescope, a microscope or a binocular, an eyepiece lens adapted for observing an object image formed by the objective lens, and a telescope or a binocular (an observing optical system) using the objective lens and the eyepiece lens.
2. Description of Related Art
An observing optical system, such as a telescope or a binocular, is provided with an eyepiece lens for making a real image formed by an objective lens observable as an enlarged virtual image. When the observer views the virtual image, a field of view observable by the observer is enlarged according to the magnification of the observing optical system defined by the ratio of the focal length of the objective lens to the focal length of the eyepiece lens. Accordingly, the eyepiece lens is required to have the various aberrations, particularly, curvature of field, lateral chromatic aberration and coma, corrected in an observable apparent field of view.
Further, it is desired in the observing optical system that the eye relief (the distance from the last lens surface to the eye point) is sufficiently long in such a way as to enable the observer to observe an image with spectacles on.
In Japanese Laid-Open Patent Application No. Hei 09-090246 (corresponding to U.S. Pat. No. 5,748,380), there is proposed an eyepiece lens in which an apparent field of view is 60xc2x0 or more and the eye relief is longer than the focal length of the whole eyepiece lens.
In the eyepiece lens proposed in the above Japanese Laid-Open Patent Application No. Hei 09-090246, a negative cemented lens is disposed on the object side of an intermediate image of an objective lens, and a positive cemented lens and a plurality of positive lenses are disposed on the observer side of the intermediate image, for the purpose of correcting the various aberrations while keeping the eye relief long.
In addition, for example, in Japanese Laid-Open Patent Application No. Hei 05-119273 (corresponding to U.S. Pat. No. 5,255,121), there is disclosed an eyepiece lens having the eye relief sufficiently secured and the various aberrations corrected well. This eyepiece lens, although having a sufficiently wide angle of view and the various aberrations corrected well, is designed for use in a microscope and, therefore, has the long focal length and also the long total length.
In general, in an eyepiece lens, if an apparent field of view is widened to a visual angle of 60xc2x0 or thereabout and a compact lens arrangement is adopted while enlarging the distance from the eyepiece lens to the pupil position of the observer (eye relief or eye distance), the number of refractive lens surfaces having convergency relative to an off-axial principal ray would increase, so that it becomes difficult to correct the various aberrations well to obtain high optical performance.
For example, in the case of an eyepiece lens according to numerical example 1 set forth in the above-mentioned Japanese Laid-Open Patent Application No. Hei 09-090246, curvature of field, if reduced to aberration in an afocal system, becomes +1.5 Dptr (diopter) with regard to a sagittal plane and xe2x88x923 Dptr with regard to a meridional plane at the most marginal portion of a field of view, and astigmatism becomes 4 Dptr. Therefore, in a case where the eyepiece lens is used for the actual observing optical system, when the observer brings a central part of an observation object into focus, the observer would sense image blurring or image flowing at the marginal portion of a field of view. Further, the total length of the eyepiece lens is such large as six times the focal length of the entire eyepiece lens.
In general, it is very difficult to obtain an eyepiece lens capable of, while securing the sufficient eye relief, correcting aberration sufficiently and reducing the total lens length, i.e., making the reduction in size and the high optical performance compatible with each other.
In the meantime, in general, an objective lens for use in a telescope, a binocular or the like is required, at the time of designing, to have the various aberrations, such as spherical aberration, chromatic aberration and coma, suppressed to sufficient small degrees.
FIGS. 30 and 31 are sectional views respectively showing optical systems of objective lenses which are generally used, as viewed in such a way as to include an optical axis.
In FIG. 30, reference numeral 7 denotes an objective lens, reference numeral 8 denotes an erecting prism serving as an image inverting means, reference numeral 9 denotes an optical axis, and reference numeral 10 denotes an image plane on which an object image is formed by the objective lens 7. As shown in FIG. 30, the objective lens 7 is composed of a combination of a positive lens 7a and a negative lens 7b. This arrangement is adopted for the purpose of utilizing the difference in dispersion between glass materials of the respective lenses so as to correct the various aberrations, particularly, longitudinal chromatic aberration and spherical aberration, which appear along the optical axis.
The objective lens shown in FIG. 31 is an example of the so-called telephoto type, in which a negative lens unit 12 is disposed behind a positive lens unit 11. In FIG. 31, in addition to the positive lens unit 11 and the negative lens unit 12, reference numeral 13 denotes an erecting prism, reference numeral 14 denotes an optical axis, and reference numeral 15 denotes an image plane.
In a case where an objective lens is intended to have much higher optical performance, particularly, to correct well chromatic aberration, which appears along the optical axis, so as to have high optical performance, a glass material having a large Abbe number (low dispersion) and having extraordinary dispersion is used as the material of a positive lens constituting a part of the objective lens. As such a glass material having extraordinary dispersion, there are, by way of example, s-FPL51 (trade name) manufactured by K. K. OHARA, FK01 (trade name) manufactured by SHOTT Inc., etc. With such a glass material adopted, it is possible to improve chromatic aberration, etc., greatly.
Further, the magnification of a binocular or a telescope is determined by the ratio of the focal length of the objective lens to the focal length of the eyepiece lens. Accordingly, in order to increase the magnification of the binocular or the telescope, there are adopted a method of lengthening the focal length of the objective lens or a method of shortening the focal length of the eyepiece lens. In the case of the method of shortening the focal length of the eyepiece lens, the diameter of an exit pupil would be decreased.
In a case where the focal length of the objective lens is lengthened with the same lens construction kept, the total length of the objective lens would be increased by the amount of the focal length. In order to lengthen the focal length and shorten the total length, there is, in many cases, adopted the so-called telephoto type, in which a negative lens unit is disposed behind a positive lens unit.
As such a telephoto type, there is an ultratelephoto objective lens disclosed in Japanese Patent Publication No. Sho 63-55045, which is an objective lens having an aperture ratio of 1:7 or thereabout.
In a case where such a telephoto type is used for the objective lens, since a positive lens included in the first lens unit has a large refractive power, longitudinal chromatic aberration tends to occur more greatly. In general, in order to suppress this chromatic aberration, there is adopted a method of using a glass material having extraordinary low dispersion.
The ultratelephoto objective lens disclosed in Japanese Patent Publication No. Sho 63-55045 is so relatively dark as to have an aperture ratio of 1:7 or thereabout. If an objective lens is used for a binocular and the binocular is intended to be reduced in size, the objective lens is desirable to be a brighter lens system. Further, if the objective lens is intended to be reduced in size, it is preferable to adopt a telephoto-type optical system for the objective lens. In this instance, in order to lengthen the focal length of the objective lens, the second lens unit of negative refractive power is disposed at a distance from the first lens unit of positive refractive power.
In such a lens construction, the sensitivity due to the decentration of lenses becomes high, so that a structural arrangement for holding the lenses is required to have high accuracy. Further, if a lens system is intended to be reduced in size by strengthening a refractive power of each lens unit, the occurrence of the various aberrations would become large, causing such a tendency as to lower optical performance.
In view of the above conventional arrangement, it is a first object of the invention to provide a small-sized eyepiece lens capable of correcting well the various aberrations over the entire observation filed of view to obtain a high-image-quality observation image, while having an apparent visual angle, i.e., a wide field of view, and, moreover, having a long eye distance.
Further, it is a second object of the invention to provide an objective lens having such a small F-number as to be adapted for a telescope or a binocular, and capable of correcting well the various aberrations to obtain good optical performance, while reducing the size of a lens system.
To attain the above objects, in accordance with an aspect of the invention, there is provided an eyepiece lens, which comprises, in order from an entrance side for light to an exit side for light, a first lens unit of negative refractive power, the first lens unit consisting of one negative lens having a concave surface facing the entrance side, which is stronger in refractive power than a surface thereof facing the exit side, a second lens unit of positive refractive power, the second lens unit consisting of one positive meniscus lens having a convex surface facing the exit side, a third lens unit of positive refractive power, the third lens unit consisting of a cemented lens of meniscus form having a convex surface facing the exit side and of positive refractive power as a whole, a fourth lens unit of positive refractive power, the fourth lens unit consisting of a positive lens having a surface facing the exit side which is stronger in refractive power than a surface thereof facing the entrance side, and a fifth lens unit of positive refractive power, the fifth lens unit having a surface facing the entrance side which is stronger in refractive power than a surface thereof facing the exit side, wherein an intermediate image by an objective lens is formed between the second lens unit and the third lens unit.
In addition, in accordance with another aspect of the invention, there is provided an objective lens, which comprises, in order from an object side to an image side, a first lens unit of positive refractive power, the first lens unit consisting of one positive lens, and a second lens unit of negative refractive power, the second lens unit consisting of one positive lens and one negative lens, wherein the objective lens satisfies the following conditions:
0.5xe2x89xa6f1o/foxe2x89xa60.9 
1.1xe2x89xa6xe2x88x92f2o/foxe2x89xa62.5 
0.1xe2x89xa6D12o/foxe2x89xa60.2
where fo is the focal length of the entire objective lens, f1o is the focal length of the first lens unit, f2o is the focal length of the second lens unit, and D12o is an interval between the first lens unit and the second lens unit.
In addition, in accordance with a further aspect of the invention, there is provided an objective lens, which comprises, in order from an object side to an image side, a first lens unit of positive refractive power, the first lens unit consisting of one positive lens, a second lens unit of negative refractive power, the second lens unit consisting of one positive lens and one negative lens, and an erecting prism for erecting an image, wherein the objective lens satisfies the following conditions:
0.5xe2x89xa6f1o/foxe2x89xa60.9 
1.1xe2x89xa6xe2x88x92f2o/foxe2x89xa62.5 
1.6xe2x89xa6Np 
where fo is the focal length of the entire objective lens, f1o is the focal length of the first lens unit, f2o is the focal length of the second lens unit, and Np is a refractive index of material of the erecting prism.
In addition, in accordance with a still further aspect of the invention, there is provided an optical apparatus, such as a telescope or a binocular, which comprises the eyepiece lens or the objective lens according to the above aspects.
In addition, in accordance with a still further aspect of the invention, there is provided a binocular, which comprises, in order from an entrance side for light to an exit side for light, a pair of objective lenses, the pair of objective lenses respectively forming images, a pair of erecting prisms, the pair of erecting prisms erecting the images formed respectively by the pair of objective lenses, and a pair of eyepiece lenses, the pair of eyepiece lenses being provided for observing the images erected respectively by the pair of erecting prisms, wherein the binocular satisfies the following condition:
1.6 less than Nep 
where Nep is a refractive index of material of the pair of erecting prisms.
These and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.